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Earth’s Major Spheres The view of Earth shown in Figure 5B should help you see why the physical environment is traditionally divided into three major spheres: the water portion of our planet, the hydrosphere; Earth’s gaseous envelope, the atmosphere; and the geosphere. Our environment is characterized by the continuous interactions of air and rock, rock and water, and water and air. The biosphere, which is made up of all the life-forms on Earth, interacts with all three of these physical spheres. Earth can be thought of as consisting of four major spheres: the hydrosphere, atmosphere, geosphere, and biosphere. Hydrosphere Water is what makes Earth unique. All of the water on Earth makes up the hydrosphere. Continually on the move, water evaporates from the oceans to the atmosphere, falls back to Earth as rain, and runs back to the ocean. The oceans account for approximately 97 percent of the water on Earth. The remaining 3 percent is fresh water and is present in groundwater, streams, lakes, and glaciers. Although these freshwater sources make up a small fraction of the total amount of water on Earth, they are quite important. Streams, glaciers, and groundwater are responsible for sustaining life and creating many of Earth’s varied landforms. Atmosphere A life-sustaining, thin, gaseous envelope called the atmosphere surrounds Earth. It reaches beyond 100 kilometers above Earth, yet 90 percent occurs within just 16 kilometers of Earth’s surface. This thin blanket of air is an important part of Earth. It provides the air that we breathe. It protects us from the sun’s intense heat and dangerous radiation. The energy exchanges that continually occur between space, the atmosphere, and Earth’s surface produce weather and climate. If Earth had no atmosphere, life on our planet as we know it could not exist. Many of the processes and interactions that make the surface such a dynamic place would not occur. For example, without weathering and erosion, the face of our planet might more closely resemble the moon. Geosphere Lying beneath both the atmosphere and the ocean is the geosphere. Because the geosphere is not uniform, it is divided into three main parts based on differences in composition—the core, the mantle, and the crust. Figure 6A shows the dense or heavy inner sphere that is the core; the less dense mantle; and the lighter, thin crust. The crust is not uniform in thickness. It is thinnest beneath the oceans and thickest beneath the continents. Biosphere The biosphere includes all life on Earth. It is concentrated in a zone that extends from the ocean floor upward for several kilometers into the atmosphere. Plants and animals depend on the physical environment for life. However, organisms do more than just respond to their physical environment. Through countless interactions, organisms help maintain and alter their physical environment. Without life, the makeup and nature of the solid Earth, hydrosphere, and atmosphere would be very different.